1. Field of the Invention
The present invention relates to a signal transmitting system. More particularly, the present invention relates to a system for transmitting a signal having relatively high frequency components wherein a spurious radiation from the signal source can be effectively attenuated.
2. Description of the Prior Art
FIG. 1 is a schematic block diagram showing a general hardware structure of a product utilizing a microcomputer, for illustrating the background of the present invention. This apparatus 1 comprises a power supply circuit 20 connected to an alternating current power source, such as commercial power line, through a plug 10 and a power source line 11. The power supply circuit 20 comprises a well-known voltage transforming means (transformer), a direct current power supply circuit and the like, not shown. The circuit 20 supplies direct current power to a power source line 21 and it also supplies alternating current to a power source line 22. Power on line 21 operates a digital circuit 30. The digital circuit 30 comprises a circuit board (not shown) on which a microcomputer and an associated circuit are mounted. As is well known, the microcomputer includes a combination of a read only memory (ROM), a random access memory (RAM), an arithmetic logic unit (ALU) and an I/O port. An input port of the digital circuit (microcomputer) 30 is connected to an input device 40 through a signal line 31. The input device 40 comprises a keyboard, a manual switch and the like to apply their operation signal (ON/OFF signal) to the digital circuit (microcomputer) 30 as an input signal. An output port of the digital circuit 30 applies an output signal (for example, a digital signal and or analog signal such as a video signal) to an output device 50 through the signal line 32. The output device 50 includes a cathode ray tube (CRT) display and a sound circuit, not shown, to output a picture or a sound in response to a signal applied from the digital circuit 30 through the signal line 32.
The digital apparatuses as shown in FIG. 1, particular, products utilizing a microcomputer, are widespread and are even used in the home already. However, a spurious radiation of electromagnetic waves generated by such digital circuits has caused certain problems. More particularly, with an increase in the processing speed in a recent microcomputer and the high speed of electronic circuits, a spurious electromagnetic wave of up to several hundred MHz above an audio frequency is radiated into the air by various dividing signals and higher harmonic components with respect to a basic frequency of several HMz. Accordingly, on the grounds that such spurious radiation interferes with general radio communication or is an obstacle to commercial communication such as television broadcasting, considerably strict restrictions to such spurious radiation will be enacted in many countries, for example, by the FCC and the FTZ.
Conventionally, some approaches are known for preventing spurious radiation in the apparatus shown in FIG. 1, for example. One approach is to house a circuit board (on which a microcomputer is mounted) which serves as a signal source in a shielded casing. However, if only such a signal source is housed in a shielded casing, there is almost no effect because a power supply line and a signal line all act as antennas if and when a power supply and input-output devices are connected to the signal source. Another approach is to house all of the power supply circuits and other apparatuses in a shielded casing. However, if a power supply circuit and other apparatuses are all housed in a shielded casing, the mechanism becomes complicated and large in size. Hence, the cost thereof becomes too expensive, and the work efficiency is remarkably decreased. Therefore, the following approaches are proposed as further approaches for a power source line and a signal line that are necessarily exposed from the shielded casing to the exterior. That is, such approaches are, for example, to bypass high frequency components by making the signal line pass through a feed-through capacitor or to relay such a signal line by using a connector, having a special bypass capacitor, for removing high frequency components. However, in accordance with such approach attempting to prevent spurious radiation by removing such high frequency components, a necessary information signal is adversely affected by such a feed-through capacitor and special connector, and hence, there has been a problem in that the quality of, for example, video signals and other synchronization signals or clock signals are remarkably deteriorated. In addition, a frequency spectrum which is to prevent spurious radiation must be as widely spread as described in the foregoing and the number of required signal lines differs widely depending on the situation. For this reason, there are few components suitable for this purpose, which causes a further cost increase. Furthermore, although the power source line and signal line are shielded by a metal pipe or a special shielded cable, such an approach using a metal pipe or a shielded cable causes a decrease of work efficiency and an increased cost.